American Mineralogist, Volume 92, pages 1492–1501, 2007 The crystal structure of pyroxenes along the jadeite–hedenbergite and jadeite–aegirine joins F. NESTOLA,1,* M. TRIBAUDINO,2 T. BOFFA BALLARAN,1 C. LIEBSKE,3 AND M. BRUNO4 1Bayerisches Geoinstitut, Universität Bayreuth, Universitätstrasse 37, D-95447, Bayreuth, Germany 2Dipartimento di Scienze della Terra, Università di Parma, Parco Area delle Scienze 157/A, I-43100, Parma, Italy 3Institute for Mineralogy and Petrology, ETH Zurich, Clausiusstrasse 25, CH-8092, Zurich, Switzerland 4Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, I-10125, Torino, Italy ABSTRACT The crystal-structures of seven synthetic pyroxenes along the jadeite–hedenbergite (Jd57Hd43, Jd26Hd74, Jd0Hd100) and jadeite–aegirine (Jd100Ae0, Jd76Ae24, Jd35Ae65, Jd0Ae100) joins were reÞ ned using data col- lected by means of single-crystal X-ray diffraction (space group C2/c, R4σ between 2.2 and 3.4%). The M2 and M1 polyhedral volumes and bond lengths increase with increasing aegirine and heden- bergite content, moreover the Ca for Na substitution along the jadeite–hedenbergite join changes the M2 coordination from 6 + 2 to 4 + 4, with remarkable tilting of the tetrahedral chains. The value of the displacement parameters follows the trend UeqM2 > UeqO2 > UeqO3 > UeqO1 > UeqM1 ≈ UeqT for all samples belonging to the jadeite–aegirine join and for pure hedenbergite; in contrast,, for pyroxenes with intermediate compositions between hedenbergite and jadeite the trend is UeqO1 > UeqO2 > UeqM2 > UeqO3 > UeqM1 ≈ UeqT, with O1 and O2 having anomalously large displacement parameters, probably due to different local structural conÞ guration around the cations with different size and charge. Cation substitution at the M1 site of Na-pyroxenes gives rise to a different structural deforma- 3+ 2+ tion with respect of the double substitution at both the M1 and M2 sites in (Na,Ca)(M ,M )Si2O6 pyroxenes as the rigid tetrahedral chains try to accommodate both the increasing size of the M1 site and the different coordination requirement of the M2 site. Keywords: Single crystal, X-ray diffraction, crystal-structure, clinopyroxenes INTRODUCTION along the aegirine–hedenbergite, jadeite–hedenbergite, and jade- Pyroxenes are among the most common minerals in the ite–aegirine joins. Structural data obtained from single-crystal crust and the upper mantle and are major constituents in several X-ray diffraction are limited to the end-members (Cameron et meteorites; knowledge of their structural and thermodynamic al. 1973; Rossi et al. 1983; Boffa Ballaran et al. 1998; Heuer et properties is hence important in petrological modeling. In par- al. 2005; Redhammer et al. 2000), whereas reÞ nements of both ticular, Na-bearing pyroxenes are key phases in high-pressure single-crystal and powder data are available for several synthetic and extreme pressure assemblages, up to more than 20 GPa and pyroxenes along the aegirine–hedenbergite join (Redhammer 1600 °C (Tutti et al. 2000). Natural Na-bearing pyroxenes are et al. 2000, 2006). For the aegirine–jadeite solid solution only unit-cell parameters are reported (Liu and Bohlen 1995; Nestola solid solutions of jadeite (Jd, NaAlSi2O6) with aegirine (Ae, 3+ et al. 2006). NaFe Si2O6), diopside (Di, CaMgSi2O6), and hedenbergite 2+ Direct information on samples with mixed compositions (Hd, CaFe Si2O6). The end-members display C2/c space group, whereas natural omphacitic pyroxenes in eclogitic assemblages, instead of just a mere comparison between the end-members with a composition close to the binary jadeite–diopside join, has the following advantages: (1) Cation substitution seldom display a transformation from C2/c to P2/n at intermediate occurs with fully ideal behavior. Clustering of likely cations and compositions, related to coupled Ca/Na and Al/Mg cation order- order-disorder behavior may occur in intermediate samples, af- ing. Several structural and crystal chemical investigations were fecting structural and thermodynamic properties. Local structural performed on the binary solid solutions of aegirine–jadeite and behavior at a short-range scale is averaged in single structure diopside–jadeite (e.g., Popp and Gilbert 1972; Carpenter et al. investigation, but may be revealed by the presence of anomalous 1990a, 1990b; Holland 1983; Gasparik 1985; Boffa Ballaran et atomic displacement parameters in the average structure for al. 1998). However, very few studies are available on pyroxenes intermediate compositions (e.g., Tribaudino and Nestola 2002). (2) The structural analysis along a given join clariÞ es the mecha- nism of cation substitution. Moreover, subtle structural changes, * Present address: Dipartimento di Geoscienze, Università di which could be overlooked analyzing only the end-members, can Padova, Corso Garibaldi 37, I-35137, Padova, Italy. E-mail: be enhanced by the analysis of high quality structural data for [email protected] intermediate compositions. 0003-004X/07/0809–1492$05.00/DOI: 10.2138/am.2007.2540 1492 NESTOLA ET AL.: CRYSTAL STRUCTURES OF Na-PYROXENES 1493 The structure of seven synthetic pyroxenes along the jade- starting from the atomic coordinates of Cameron et al. (1973). The atomic scat- ite–aegirine and hedenbergite–jadeite joins was studied, to clarify tering curves were taken from the International Table for X-ray Crystallography (Ibers and Hamilton 1974). Ionized scattering factors were used for the cations the structural changes occurring for the isovalent substitution of at the M1 and M2 sites, whereas neutral vs. ionized scattering factors were used 3+ Fe for Al along the jadeite–aegirine join and for the coupled for the oxygen and Si atoms. Final occupancies were obtained following the same heterovalent substitution of CaFe2+ for NaAl along the jade- reÞ nement procedure used in several works on cation partitioning of Na-clinopy- ite–hedenbergite join. The major aim is to outline a generalized roxenes (Rossi et al. 1983; Oberti and Caporuscio 1991; Boffa Ballaran et al. 1998). – 3 mechanism of structural deformation for pyroxenes with cation During the reÞ nement of the hedenbergite sample a residual of 1.72 e /Å in the difference Fourier map suggested the presence of a M2' split position occupied by substitutions at both the M1 and M2 sites. Fe2+ as observed in previous studies (Bruno et al. 1982; Rossi et al. 1987; Oberti This work is a part of a wider investigation on structural, and Caporuscio 1991; Boffa Ballaran et al. 1998; Tribaudino and Nestola 2002; compressional and thermal properties for pyroxenes belonging Nestola and Tribaudino 2003). Further reÞ nements were, therefore, performed with to jadeite–aegirine and jadeite–hedenbergite solid solutions. an isotropic M2' position and with the constraint Ca (M2) + Fe2+ (M2') = 1. Cation occupancies obtained by crystal structure reÞ nement for all samples are reported in Table 1b. Unit-cell parameters, data collection and reÞ nement details, EXPERIMENTAL METHODS fractional atomic coordinates, anisotropic thermal parameters, and polyhedral bond Single crystals along the Jd–Ae and Jd–Hd joins were synthesized using lengths and volumes are reported in Tables 2, 3, and 4, respectively. multianvil presses at the Bayerisches Geoinstitut Bayreuth and at the ETH Zürich. Synthesis experiments were carried out at a pressure of 6 GPa, at temperatures RESULTS between 1300 and 1400 °C and with run times of 5–8 h. Initial starting materials were stoichiometric mixtures of oxides and salts (CaCO , SiO , Na SiO , Fe O , 3 2 2 3 2 3 Unit-cell parameters Al2O3), subsequently melted between 1200 and 1300 °C and then quenched to glasses. For the jadeite–hedenbergite compositions it was also necessary to reduce The unit-cell parameters along the jadeite–aegirine and 3+ 2+ the Fe of the glasses starting materials to Fe , therefore, these were annealed jadeite–hedenbergite joins are shown in Figure 1 as a function for 24 h at 600 °C in a controlled atmosphere (CO2/H2). Details relative to the synthesis and compositions of the crystals along the jadeite–aegirine join are of composition. In both systems, a decrease in jadeite component reported by Nestola et al. (2006). The microprobe analyses for the samples along leads to a non-linear increase of the a and b cell parameters (Figs. the jadeite–hedenbergite join are reported in Table 1a. For these three pyroxenes 1a and 1b). The c parameter (Fig. 1c) increases linearly along the analyses were performed at the “Istituto di Geoscienze e Georisorse,” CNR the jadeite–aegirine, although the absolute increase is not as Padova (Italy) using a CAMECA-CAMEBAX electron microprobe operating with a Þ ne-focused beam (~1 μm) at an acceleration voltage of 15 kV and a beam current pronounced as that of a and b. Along the jadeite–hedenbergite of 15 nA in wave-length dispersive mode (WDS), with 20 s counting times for both join the c parameter has a strong deviation from linearity with peak and total background. The following standards were used: NaAlSi3O8 for Na, samples at intermediate compositions having larger values than MgO for Mg, Al2O3 for Al, CaSiO3 for Si and Ca, and Fe2O3 for Fe. the end-members (Fig. 1c). The β angle is constant along the Intensity data were collected using a Kappa geometry Xcalibur diffractometer jadeite–aegirine join, while it decreases non-linearly along the with graphite monochromated MoKα radiation (5 ≤ 2θ ≤ 80°) in an ω-scan mode with a continuous-integrative step scan (0.05°/s, 60 scan steps, scan width 1.2°) jadeite–hedenbergite join (Fig. 1d). As a result, the unit-cell vol- under ambient conditions. The sample-detector distance was 135 mm. The program ume increases signiÞ cantly for both joins (Fig. 1e). The volume- Win-IntegrStp (version 3.3, Angel 2003) was used to integrate the step-scan data composition trend is not linear for both the solid solutions, (see applying the Lorentz-polarization correction. The intensity data were corrected for also Nestola et al. 2006), suggesting that some non-ideality also absorption using the program ABSORB V6.0 (Angel 2004).